dt-bindings: hwrng: Add Samsung Exynos 5250+ True RNG bindings

Add binding documentation for the True Random Number Generator
found on Samsung Exynos 5250+ SoCs.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>